Urea is generally produced from ammonia and carbon dioxide. It can be prepared by introducing an ammonia excess together with carbon dioxide at a pressure between 12 and 40 MPa and at a temperature between 150° C. and 250° C. into a urea synthesis section. In urea technology this is generally referred to as “high pressure” (HP). The resulting urea formation can be presented best in the form of two consecutive reaction steps, in the first step ammonium carbamate being formed according to the exothermic reaction:2NH3+CO2→H2N—CO—ONH4 after which the ammonium carbamate formed is dehydrated in the second step to give urea according to the endothermic equilibrium reaction:H2N—CO—ONH4H2N—CO—NH2+H2O
Typical urea production plants further comprise a recovery section and a finishing section. In the recovery section non-converted ammonia and carbon dioxide are recovered and recirculated to the synthesis section. The recovery section is generally followed by an evaporation section. Therein the urea concentration is further increased by the evaporation of water, resulting in a highly concentrated solution that is generally referred to as a urea melt. In the finishing section, typically, the urea melt is brought into a desired solid, particulate form, generally involving techniques such as prilling, granulation, or pelletizing.
Melamine is regularly produced from urea according to the following reaction equation:6 (NH2)2CO→C3H6N6+6 NH3+3 CO2.
Generally two technologies exist for the production of melamine. One is a catalytic “low pressure” process, which applies atmospheric pressure (from atmospheric pressure to about 1 MPa). The other is called a “high pressure” process. In melamine production, high pressure refers to a pressure of at or above about 70 bar (7 MPa), more typically at or above 80 bar (8 MPa). The off-gases from the high pressure process generally have a pressure of from 1 to 4 MPa. It should be noted that, in urea technology, these pressures are within a range normally referred to as a “medium pressure (“MP”).
It is well-known in the art to combine the production of urea and the production of melamine. Thereby, generally, at least part or all of the urea produced in a urea plant, is sent to a melamine plant as a starting material. The production of melamine yields an off-gas comprising ammonia (NH3) and carbon dioxide (CO2) in a 2:1 stoichiometric ratio, which corresponds to that of the reactants in urea synthesis. Thus, the integrated production of urea and melamine typically involves feeding an ammonia and carbon dioxide containing gas stream obtained from the melamine plant, directly or indirectly to the synthesis section of the urea plant.
A method of the aforementioned type is disclosed in EP 1716111 B1 and is illustrated in the block diagram as given in FIG. 1. In this process the gas stream from the melamine plant is fed to an off-gas condensation section together with the carbamate formed in the recirculation section of a urea plant. This off-gas condensation section comprises at least one condenser apparatus. Upstream of the condenser apparatus, the off-gas from the melamine plant is mixed with the carbamate aqueous solution formed in the recirculation section of the urea plant. Alternatively, said off gas and carbamate aqueous solution are separately added to this condenser apparatus.
A connection, particularly a direct fluid connection, between a gaseous stream from one process system, viz. here the off-gas from a melamine plant, and another process system, viz. here the carbamate stream in the condensation section of the urea plant, is prone to cause in general a process disorder in the upstream processing of the former gas stream (i.e., the off-gas stream from the melamine plant. If the pressure in said condensation section increases, the pressure of the added off-gas from the melamine plant should increase as well, in order to obtain a positive flow. However an increase and fluctuation of the pressure in the melamine processing upstream of the-off gas, is possible only to a limited extent. Such a pressure increase causes a process disorder, and finally product losses in the melamine plant.
Further, it is noted that a urea plant normally obtains heat energy from, inter alia, the necessary compression of carbon dioxide. In the event that a urea plant is to directly process the gaseous stream formed in the melamine plant, which comprises carbon dioxide, less carbon dioxide will be subjected to compression, and thus additional heat energy will be required. This is especially the case for urea plants according the carbon dioxide stripping process. As a result, the energy consumption of the urea plant, typically expressed in steam consumption (kilogram steam per produced ton of urea product), increases by an increasing capacity of the coupled melamine facility. Hence, this provides an additional reason for the importance of improving the process economy of a system in which urea and melamine plants are coupled. It will be understood that a more stable process will generally be more economical.
Another issue in relation to systems in which an aqueous carbamate condensate is obtained and circulated to urea synthesis, relates to the amount of water. With reference to the above reaction equations for the synthesis of urea, it will be clear that the additional presence of water would shift the equilibrium to the side of the starting materials. Desirably, the amount of water is as high as necessary to have a transportable carbamate solution, but as low as possible in order to send as little water as possible to the urea synthesis section. The presence of an additional off-gas condensation section, such as in EP 1716111, therefore bears a risk of carrying unwanted additional water to the synthesis section.